The Airbus reference Getting to Grips with Aircraft Performance provides a description in section 4.1 Takeoff Flight Path.
They describe the takeoff flight path as four segments as shown in the following figure;
They provide the following discussion:
So, below 400 feet, the speed must be maintained constant to a minimum
of V2. Above 400 feet, the aircraft must fulfill a minimum
climb gradient, which can be transformed into an acceleration
capability in level flight. Therefore, the regulatory minimum
acceleration height is fixed to 400 feet above the takeoff surface.
Nevertheless, during the acceleration segment, obstacle clearance must
be ensured at any moment. Therefore, the operational minimum
acceleration height is equal to or greater than 400 feet (Figure C16).
The Maximum Acceleration Height is described as follows:
JAR/FAR 25.111 (c)(3) At each point along the takeoff flight path,
starting at the point at which the aeroplane reaches 400 ft above the
takeoff surface, the available gradient of climb may not be less than:
• 1.2% for a two-engined airplane
• 1.7% for a four-engined airplane
The Maximum Takeoff Thrust (TOGA) is certified for use for a maximum
of 10 minutes, in case of an engine failure at takeoff, and for a
maximum of 5 minutes with all engines operating. The Maximum
Continuous Thrust (MCT), which is not time-limited, can only be
selected once the enroute configuration is achieved (i.e. when the
aircraft is in clean configuration at green dot speed). As a result,
the enroute configuration (end of the third segment) must be achieved
within a maximum of 10 minutes after takeoff, thus enabling the
determination of a maximum acceleration height (Figure C16).
The Acceleration phase ends when the aircraft reaches green dot speed in a clean configuration. Based on the requirement to maintain a minimum climb gradient during this phase, the Maximum Acceleration Height will be above the Minimum Acceleration Height.
The biggest caveat in the definition is that heights and climb gradients must comply with adequate obstacle clearances.